Cyclosilicate · Uniaxial (−)
Tourmaline in thin section
Strongly pleochroic and darker when its long dimension is at right angles to the lower polarizer, the opposite of biotite. No cleavage. Medium relief. Stubby to needle-like euhedral prisms.
Plane-polarized light
| Color | Brown, Green, Blue / lavender |
|---|---|
| Pleochroism (color change on rotation) | Strong |
| Relief | Moderate |
| Cleavage | None |
| Shape | Prismatic, Needles / fibrous, Euhedral |
Cross-polarized light
| Maximum birefringence | about 0.025 |
|---|---|
| Highest interference color | 2nd-order blues, greens, 3rd-order |
| Extinction | Parallel |
| Twinning | None |
Interference colors are often masked by the mineral's own color.
Telling it apart
Biotite has a perfect cleavage and is darker when its cleavage is parallel to the polarizer. Colored apatite is also darker when parallel.
Where you'll find it
Granite, pegmatite, schist, detrital grains in sandstone
Photo examples
Alex Strekeisen · Search Wikimedia Commons
Photos open on the source sites.
Identify an unknown grain Read interference colors Practice mineral ID
Related minerals
Sources
OpticMin, ATHENA, Univ. of Geneva (P. Perroud) · Alex Strekeisen thin-section collection · Distinguishing features of minerals (UT Arlington) · MIT OCW 12.109 amphibole lab · SEPM Strata: carbonate staining · Wikipedia mineral pages (infobox optical data) · Athabasca University rocks and minerals · Smith College petrography: perthite